In the exploration of oil, gas, and geothermal energy, drilling operations are used to create boreholes, or wells, in the earth. These operations normally employ rotary and percussion drilling techniques. In rotary drilling, the borehole is created by rotating a tubular drill string with a drill bit secured to its lower end. As the drill bit deepens the hole, tubular segments are added to the top of the drill string. While drilling, a drilling fluid is continually pumped into the drilling string from surface pumping equipment. The drilling fluid is transported through the center of the hollow drill string and into the drill bit. The drilling fluid exits the drill bit at an increased velocity through one or more nozzles in the drill bit. The drilling fluid then returns to the surface by traveling up the annular space between the borehole and the outside of the drill string. The drilling fluid carries rock cuttings out of the borehole and also serves to cool and lubricate the drill bit.
The type of drill bit used to drill the well will depend largely on the hardness of the formation being drilled. One type of rotary rock drill is a drag bit. Early designs for drag bits included hard facing applied to steel cutting edges. Modern designs for drag bits have extremely hard cutting elements, such as natural or synthetic diamonds, mounted to a bit body. As the drag bit is rotated, the hard cutting elements scrape against the bottom and sides of the borehole to cut away rock.
Another type of rotary rock drill is the rolling cutter bit. These drill bits have rotatable cutters mounted on bearings on the body of the drill bit, which rotate as the drill bit is rotated. Cutting elements, or teeth, protrude from the cutters. The angles of the cutters and bearing pins on which they are mounted are aligned so that the cutters essentially roll on the bottom of the hole with controlled slippage. As the rolling cutter cutters roll on the bottom of the hole being drilled, the teeth or carbide inserts apply a high compressive load to the rock and fracture it. The cutting action of rolling cutter cutters is typically by a combination of crushing, chipping and scraping. The cuttings from a rolling cutter rock bit are typically a mixture of moderately large chips and fine particles.
There are two general types of rolling cutter drill bits; TCI bits and steel-tooth bits. In the oilfield, TCI is a well-recognized abbreviation for Tungsten Carbide Insert. These bits have steel cutters with a plurality of tungsten carbide or similar inserts of high hardness that protrude from the surface of the cutter. There are numerous styles of TCI drill bits designed for various formation hardnesses, in which the shape, number and protrusion of the tungsten carbide inserts will vary, along with cutter and journal angles.
Steel-tooth bits are also referred to as milled-tooth bits, since most bits have their steel teeth created in a milling machine. However, in larger bits, it is also known to cast the steel teeth and, therefore, “steel-tooth” is the better reference. The steel-tooth bit has cutters having an integral body of hardened steel with teeth formed on the periphery. There are numerous styles of steel-tooth drill bits designed for formations of varying hardness in which the shape, number and protrusion of the teeth will vary, along with cutter and journal angles.
The cost efficiency of a drill bit is determined by the drilling life of the drill bit and, largely, the rate at which the drill bit penetrates the earth. Under normal drilling conditions, the teeth of the steel-tooth bits are subject to continuous impact and abrasive wear because of their engagement with the rock being drilled. As the teeth are worn away, the penetration rate of the drill bit is reduced, and the cost of drilling increases significantly.
To increase the cost efficiency of a steel-tooth drill bit, it is necessary to increase the wear resistance of the steel teeth. To accomplish this, in some instances, it may be desirable to deposit one or more layers of a wear-resistant material or “hardfacing” to the exposed surfaces of the steel teeth. Fusion hardfacing refers to a group of welding related techniques that apply a wear resistant alloy to a substrate metal. Common hardfacing techniques include arc welding and gas torch welding, among other welding processes.
Conventional welding techniques used to apply hardfacing to steel-tooth drill bits include oxyacetylene welding (OAW) and atomic hydrogen welding (AHW). Currently, the only method known to be in use in the commercial production of rolling cutters for rock bits is manual welding. Cutters are mounted on a positioning table, and the welder holds a welding torch and welding rod while applying the hardfacing to the desired portions of the teeth on each cutter. The welder must manually move from tooth to tooth on the cutter while addressing the cutter from various angles during the course of this process.
Conventional hardfacing materials used to add wear resistance to the steel teeth of a rotary rock bit includes tungsten carbide particles in a metal matrix, typically cobalt or a mixture of cobalt and other similar metals. Many different compositions and formulations of hardfacing material have been employed in the rock bit field to achieve wear-resistance, durability and ease of application. Typically, these hardfacing materials are supplied in the form of a welding rod, but can be found in powder form for use with other types of torches.
The quality of a hardfacing application has several physical indicators including uniformity, thickness, coverage, porosity, and other metallurgical properties. Historically, the individual skills of the welder have largely controlled the hardfacing quality. Hardfacing quality is known to vary between rock bits, and even between cutters on a rock bit, and teeth on a cutter. Limited availability of qualified welders has aggravated the problem. The manual application is extremely tedious, repetitive, skill-dependent, time-consuming, and expensive. Indeed, application of cutter hardfacing is considered the single most tedious and skill-dependent step in the manufacture of a steel-toothed rock bit. The consistency of even a skilled welder can vary during a work day.
As stated, the prior-art means of applying hardfacing to a cutter involves continuous manual, angular manipulation of a torch over the cutter, with the cutter held substantially stationary, but rotating, on a positioning table. After hardfacing is applied to a surface of each tooth by a welder holding a torch and welding rod containing the hardfacing material, the positioning table and cutter are indexed to a new angle and position to permit application of hardfacing to a surface of the next tooth. This continues until all the cutters have been rotated 360 degrees. At that time, the angle of the table and cutter would be adjusted for application to another tooth surface or row of teeth on the cutter.
When attempts to utilize robotics to automate the welding process were made, the same configuration was used, designating the robotic arm to replace the human operator's arm and his varied movements, while leaving the cutter on the positioning table. The positioning table was wired for automatic indexing between teeth and rows.
This is the intuitive configuration and procedure, which would be expected to provide the recognized benefits of manual hardfacing for a number of reasons. First, manual and automatic torches are much lighter and easier to continuously manipulate than the heavy steel cutters with teeth protruding in all directions. Second, the cutter must be electrically grounded, and this can be done easily through the stationary positioning table. Third, gravity maintains the heavy cutter in position on the positioning table. Fourth, highly angled (relative to vertical) manipulation of the torch allows access to confined spaces between teeth, a manipulation suited to the highly articulated movement of a robotic arm.
U.S. Pat. No. 6,392,190 describes a concept of robotically hardfacing cutters on a drill bit, in which the torch is held by a robotic arm and the cutters are moved on a positioning table. In particular, this patent discloses “an automated hardfacing system useful for hardfacing roller cones. The automated system includes a robot with an arm, a positioner, and a controller which co-ordinates the alignment of the robot and the positioner. The robot holds a hardfacing torch and is capable of movement in three axes of movement. These axes are the x, y, and z axes of the Cartesian co-ordinate system. The positioner holds a roller cone and is capable of movement in at least two axes of movement. The movement includes tilting and rotation about a Cartesian axis. The hardfacing coating produced by the automated system has improved quality and consistency as compared to the one obtained by a manual process.” The disclosure of the '190 patent illustrates the concept of replacing the typical “manual welder” used in hardfacing applications with a robot for holding the torch, and essentially describes the rather obvious expedient of more than five movable axes in the system. However, U.S. Pat. No. 6,392,190 fails to provide any specific teaching directed to the critical details of the numerous obstacles that must be overcome to actually reduce to practice the science of robotically automating the hardfacing of rolling cutters. Indeed, to date, it is not known to have successfully automated the production of the application of hardfacing to rolling cutters.
One factor preventing commercial use of robotic hardfacing has been the unsatisfactory appearance of the final product when applied using robotically held torches over stationary cutters. Another factor preventing commercial use of robotic hardfacing to rolling cutters is the commercial unavailability of a material that directly compares to conventional Oxygen Acetylene Welding (OAW) welding rod materials that can be applied with commercially available Plasma Transferred Arc (PTA) torches.
Another factor preventing commercial use of robotic hardfacing is the inability to properly identify and locate individual rolling cutter designs within a robotic hardfacing system. The cutters of each bit size and style are different, and initiating the wrong program could cause a collision of the torch and part, resulting in catastrophic failure and loss. Another factor preventing commercial use of robotic hardfacing is the inability to correct the critical positioning between the torch and part in response to manufacturing variations of the cutter, wear of the torch, and buildup of hardfacing.
Still another factor preventing commercial use of robotic hardfacing has been the inability to properly access many of the areas on the complex surface of a rolling cutter that require hardfacing with commercially available Plasma Transferred Arc (PTA) torches large enough to permit application of the required material. A small form factor (profile) is required to access the roots of the teeth that are close together. However, most conventional PTA torches require large powder ports to accommodate the flow of the medium to large mesh powder required for good wear resistance. Torches with smaller nozzles have smaller powder ports that prohibit proper flow of the desired powders.
Another factor preventing commercial use of robotic hardfacing is the complexity of programming the control system to coordinate the critical paths and applications sequences needed to apply the hardfacing. For example, heretofore undisclosed in the prior art, the known torch operating parameters, materials, application sequences, and procedures used for decades in manual hardfacing operations have proven to be mostly irrelevant to robotic hardfacing of rolling cutters. A related factor preventing commercial use of robotic hardfacing is the cost and limitation of resources. A significant investment and commitment of machine time are required to create tests, evaluate results, modify equipment, and incrementally adjust the several operating parameters, and then integrate the variations into production part programs. These and several other obstacles have, until now, prevented any commercial practice of automated hardfacing of rolling cutters anywhere in the world.
Therefore, there is a need to develop a system and method for applying hardfacing to rolling cutters consistent with the highest material and application quality standards obtainable by manual welding. There is also a need to develop a system that identifies parts, selects the proper program, and provides programmed correction in response to manufacturing variations of the cutter, wear of the torch, and buildup of hardfacing. There is also a need to develop a PTA torch design capable of accessing more of the areas on a rolling cutter that require hardfacing. There is also a need to develop a hardfacing material which performance will compare favorably to conventional Oxygen Acetylene Welding (OAW) materials and flow properly through the PTA torch design.